This is hard to answer because religion in the Roman world was not like religion in the modern world in the sense of various monolithic, mutually exclusive faiths. It's also misleading to think of different religious traditions with their own adherents: the situation was much more fluid than that.

Think of it this way: to ask someone living in the Roman empire at this time "which religion do you follow?" would have been a meaningless question, as there was not a choice of religions. Most people would have practised the religious customs of their own city or ethnic group, which would undoubtedly have features borrowed from other peoples. Doing so would not be denying the validity of other people's religious customs. Participating in religious festivals was a civic duty, not an expression of a personal choice of faith. Even if you were a priest of a specific god, this would not mean that you only accepted that god, or even that you thought that "your" god was the best.

This is a generalised picture; there were exceptions. The Jews are the obvious example, as one of the main principles of the Judaism is that there is only one all-powerful God, which obviously runs counter to the polytheistic world-view common throughout much of the Roman world. There were also certain "mystery" cults, whose participants had to be initiated: these were perhaps closer to our modern conception of "religions". But still, membership of one of these "mysteries" did not prevent one from participating in public festivals as well, although they were sometimes viewed with suspicion by the authorities due to their secretive and therefore exclusionary nature.